The present invention relates to an automatic slack adjuster for a vehicle brake and more particularly to an automatic slack adjuster having a floating reference point for the adjustment mechanism controlling the running clearance or slack of the vehicle brake as the brake linings wear during use.
Vehicle brake slack adjusters for heavy duty vehicles requiring high braking power have either a manual adjustment or, more recently, an automatic adjustment. Brake lining wear causes a gradual increase in the brake cylinder piston stroke with a consequent decrease in braking efficiency between manual adjustments. In view of this and other problems attendent with manual adjustment, automatic slack adjusters have been developed to provide for automatic and continuous compensation for the wear of the brake linings.
Automatically adjusting brake levers employing brake chamber piston rod linkages utilize the relative motion between the lever and the piston rod to actuate the adjusting mechanism on each piston stroke. Many of the presently known automatic slack adjusters, as exemplified by the U.S. Pat. Nos. 3,351,164 and 3,392,810 provide a ground or stationary reference point which cooperates with a driving member in the rotatable braking lever to provide movement of the driving member upon excessive rotation of the braking lever beyond a predetermined position; the movement of the driving member acting to cause rotation of a worm meshing with a worm gear on the braking cam shaft to adjust the amount of slack in the system.
Reference is also made to U.S. Pat. No. 3,692,152 wherein the ground reference point is replaced with a rigid link pivotally mounted on the brake piston rod and pivotally secured to the free end of an arm having an annular position rotatable in the braking lever. The arm is substantially parallel with the piston rod in the brake release position; such that upon rotation of the braking lever under the impetus of the piston rod, the piston, link, braking lever and arm provide a gradually shifting parallelogram which causes rotation of the braking lever relative to the arm to actuate the adjustment mechanism. Here again, a stationary reference point, although not rigidly secured to ground, is utilized for the actuating mechanism.
The present invention, however, has the purpose of employing the piston rod connected to the braking lever as an attachment means acting to control the movement of the reference point and thus provide a floating point which will be accurately returned to a reference location upon retraction of the braking lever and piston. To accomplish this feature, a flexible strut member is utilized extending between the piston rod and the reference arm for the adjusting mechanism. The strut member is rigidly attached at its opposite ends to the piston rod at the clevis connection with the braking lever and to the outer end of the actuating arm, such that the strut will be in an unstressed condition when the piston rod is retracted to the brake release position, and the point of connection between the strut and actuating arm becomes the reference point for the adjusting mechanism.
The present system utilizing a flexible strut member extending between the piston rod and control arm will cause the reference point established at the free end of the control arm to float during a stroking or brake engaging operation but will restore the point very closely to a predetermined position at the critical clearance sensing portion of travel, i.e. just before arrival at full-off or brake release near the end of the release stroke. The system recognizes that the reference point can float any amount except at the critical moment and provides a linkage that achieves the desired reference point of the system at rest.
The flexible strut member can accommodate offsets and variable lever lengths and will function without being jammed by ice, dirt or rocks. The strut member may be in the form or material that will act in tension and compression to achieve the desired result and will resist corrosion.
Further objects are to provide a construction of maximum simplicity, efficiency, economy and ease of assembly and operation, and such further objects, advantages and capabilities as will later more fully appear and are inherently possessed thereby.